Non-neuronal cholinergic mechanisms and the role played by these mechanisms in the development, maintenance and regulation of spontaneous myocardial rhythm will be further investigated using cultured embryonic chick heart cells, intact embryonic heart tissue in various stages of development and mature myocardium in various states of denervation. The objective of the project is to further substantiate the importance of non-neuronal mechanisms in normal cardiac rhythm and to elucidate their possible involvement in arrhythmogenesis and in the therapeutic actions of certain antiarrhythmic drugs, including the newer agent, bretylium. Data will be obtained by observing neurohormonal-drug interaction and agonist-antagonist interaction in relation to initiation or suppression of spontaneous contractility, as well as changes in spontaneous contractile frequency of cultured cells. Biochemical procedures will reveal ionic- and drug-induced changes in membrane transport processes, levels of endogenous hormonal substances and the biogenesis, turnover, storage and release of these substances in cultures and in intact myocardial preparations. The observed biochemical changes will be correlated with alterations in rhythmicity in all preparations to further test the hypothesis that endogenous intracellular non-neuronal acetylcholine acts in the regulation of spontaneous myocardial rhythm and that alterations of this physiologic function is involved in the generation and/or correction of certain cardiac arrhythmias.